Antimicrobial articles and compositions made from styrene butadiene rubber

ABSTRACT

This invention relates to certain non-silicone vulcanized rubber articles that are made from at least a majority by weight of vulcanized styrene butadiene rubber that include silver-based compounds to provide highly desirable long-term antimicrobial characteristics within the cured rubber articles. Such articles are in either solid or blown (foam or sponge) state (or combinations of both in multilayered forms) and can be utilized in a variety of different applications. This invention encompasses the presence of non-sulfur-based curing systems and agents, such as peroxide curing, that permit vulcanization and do not irreversibly bind silver ions thereto, thereby resulting in long-term antimicrobial performance of the ultimate rubber article itself. This invention also encompasses certain non-silicone pre-vulcanized raw rubber formulations made from at least a majority by weight of styrene butadiene rubber that include silver-based components to provide highly desirable long-term antimicrobial characteristics within the ultimate cured rubber articles made therefrom.

FIELD OF THE INVENTION

[0001] This invention relates to certain non-silicone vulcanized rubberarticles that are made from at least a majority by weight of vulcanizedstyrene butadiene rubber (SBR) that include silver-based compounds toprovide highly desirable long-term antimicrobial characteristics withinthe cured rubber articles. Such articles are in either solid or blown(foam or sponge) state (or combinations of both in multilayered forms)and can be utilized in a variety of different applications.

[0002] As silver-based compounds are deleteriously affected byutilization of standard curing agents and curing accelerators, such assulfur-based compounds and/or systems, the ability to provide such aneffective antimicrobial vulcanized rubber article is rather difficult.However, this invention encompasses the presence of differentnon-sulfur-based curing systems and agents, such as peroxide curing,that permit vulcanization and do not irreversibly bind silver ionsthereto, thereby resulting in long-term antimicrobial performance of theultimate rubber article itself. The rubber articles must also comprisefillers and may also include plasticizers to provide desiredcharacteristics of dimensional stability, stiffness, flexural modulus,tensile strength, abrasion resistance, elongation, and the like, for theultimate rubber article, while simultaneously and surprisingly enhancingthe control of antimicrobial efficacy of the rubber article as well.This invention also provides a simple method of producing such anantimicrobial vulcanized rubber-containing article. Furthermore, thisinvention encompasses certain non-silicone pre-vulcanized raw rubberformulations made from at least a majority by weight of styrenebutadiene rubber that include silver-based components to provide highlydesirable long-term antimicrobial characteristics within the ultimatecured rubber articles made therefrom.

DISCUSSION OF THE PRIOR ART

[0003] All U.S. patents listed below are herein entirely incorporated byreference.

[0004] There has been a great deal of attention in recent years given tothe hazards of bacterial contamination from potential everyday exposure.Noteworthy examples of such concerns include the fatal consequences offood poisoning due to certain strains of Escherichia coli being foundwithin undercooked beef in fast food restaurants; Salmonella enteritidiscontamination causing sicknesses from undercooked and unwashed poultryfood products; and illnesses and skin infections attributed toStaphylococcus aureus, Klebsiella pneumoniae, yeast (Candida albicans),and other unicellular organisms. With such an increased consumerinterest in this area, manufacturers have begun introducingantimicrobial agents within various everyday products and articles. Forinstance, certain brands of cutting boards, shoe soles, shoe inserts,medical devices and implements, liquid soaps, etc., all containantimicrobial compounds. The most popular antimicrobial for sucharticles is triclosan. Although the incorporation of such a compoundwithin liquid or certain polymeric media has been relatively simple,other substrates, specifically vulcanized rubber and surfaces thereof,have proven less accessible. Furthermore, such triclosan additives haveproven to be difficult in use or ineffective for certain bacteria. Forinstance, triclosan itself migrates easily within and out of certainpolymeric substrates and/or matrices (and thus is not very durable),lacks thermal stability (and thus readily leaches out of rubber and likematerials at higher temperatures), and does not provide a wide range ofbacterial kill (for instance does not exhibit any kill for Pseudomonasaeruginosa).

[0005] Antimicrobial rubber formulations are certainly highly desiredfor the production of vulcanized rubber articles and compositions toprovide not only antibacterial benefits, but also antifungal,antimildew, antistaining, and odor control properties. Rubber articlesare utilized in many different applications, from automobiles (hoses,tires, bumpers, etc.), to household items (toys, sink washers, gaskets,appliances, floor mats, door mats, carpeted rubber mats, gloves, and thelike), and other areas in which bacterial growth is a potential problem.Thus, there remains a long-felt need to provide an effective, durable,reliable antimicrobial pre-vulcanized rubber formulation which willprovide such long-term antimicrobial, antifungal, etc., effects withinthe final vulcanized article. Unfortunately, such a highly desiredantimicrobial rubber formulation and/or vulcanized article containingsilver-based antimicrobial agents has heretofore not been provided bythe pertinent prior art.

[0006] The closest art includes Japanese Patent Application 1997-342076which discloses the production of unvulcanized rubber formulations andarticles exhibiting antibacterial properties due to the presence ofsilver complexes. Such formulations are formed through high temperaturekneading in an oxygen-free atmosphere and are used as parts in a waterdisinfection system. Again, no vulcanized rubber is taught or obtainedwithin or through this disclosure.

[0007] Antimicrobial rubber bands have been taught in Japanese PatentApplication 1997-140034 in vulcanized form with silver antimicrobialstherein. However, such compounds are rather limited in use and thevulcanization step must include a sulfur curing agent to effectuate thefinal vulcanized arrangement of the subject rubber. Such sulfur curingagents have a remarkably deleterious effect on certain silver-basedantimicrobials such that the sulfur reacts with the silver ion to fromsilver sulfide, thereby rendering it ineffective as a bactericide. Assuch, the utilization of such specific rubber band formulations for andwithin large-scale antimicrobial articles is basically unworkable.

[0008] Certain types of antimicrobial peroxide-catalyst vulcanizedrubber formulations have been produced in the past; however, suchperoxide-cured rubbers are all silicone-based. It is well understood andaccepted that silicone rubbers cannot be vulcanized by typicalsulfur-based catalysts. Thus, the antimicrobial rubber formulations ofJapanese Patent Applications 1997-026273 and 1995-065149 as well as U.S.Pat. No. 5,466,726 are standard vulcanized silicone rubber formulationsand articles which also include certain antimicrobial compounds.

[0009] Furthermore, rubber latexes (non-vulcanized) comprisingantimicrobials have been disclosed (U.S. Pat. No. 5,736,591, forexample), as have floor mats having silver-based antimicrobialsincorporated within pile fiber components. These floor mats havenon-antimicrobial rubber backings cured through peroxide-catalyzedvulcanization to protect the pile fiber antimicrobial compounds fromattack by any sulfur compounds (as in Japanese Patent Applications1993-3555168 and 1995-38991). Again, however, to date there have been nodisclosures or suggestions of producing a vulcanized non-silicone rubberformulation or article exhibiting excellent antimicrobial propertiesthrough the long-term effective utilization of silver-basedantibacterial compounds. This invention fills such a void.

OBJECT OF THE INVENTION

[0010] It is therefore an object of this invention to provide anantimicrobial vulcanized styrene butadiene rubber-containing articleexhibiting sufficient antimicrobial activity and structural integrity towithstand repeated use without losing an appreciable level of eitherantimicrobial power or modulus strength. Another object of the inventionis to provide an antimicrobial vulcanized styrene butadiene rubber (SBR)article comprising silver-based antimicrobial compounds which includeperoxide curing agents which do not deleteriously effect theantimicrobial activity of the finished vulcanized SBR article (and thusis essentially free from sulfur-based curing agents and accelerators).Yet another object of this invention is to provide a vulcanized styrenebutadiene rubber-containing article that exhibits log kill rates forStaphylococcus aureus and Klebsiella pneumoniae (and/or other types ofbacteria as well) of at least 1.0 after 24 hours exposure at roomtemperature. Still another object of this invention is to provide avulcanized styrene butadiene rubber-containing article comprisingstructural integrity filler components and plasticizers (such as silica,metal salts, organic salts, pigments (such as carbon black), calciumcarbonate, paraffinic oils, phthalate oils, metal oxides, and the like)that also provide enhancements in the control of antimicrobial efficacyof the article itself through regulated silver ion release to thearticle surface (e.g., exhibits higher log kill rates for Staphylococcusaureus and Klebsiella pneumoniae). Another object of the invention is toprovide a finished article that exhibits increases in antimicrobialactivity after industrial washing and/or abrasion. Still another objectis to provide an antimicrobial pre-vulcanized styrene butadiene rawrubber formulation that ultimately provides a vulcanized rubber articleof sufficient antimicrobial activity and structural integrity towithstand repeated use without losing an appreciable level of eitherantimicrobial efficiency or modulus strength. Yet another object is toprovide a simple method of producing such an antimicrobial vulcanizedstyrene butadiene rubber-containing article.

[0011] Accordingly, this invention encompasses a dimensionally stablevulcanized styrene butadiene rubber-containing article exhibiting logkill rates for Staphylococcus aureus and Klebsiella pneumoniae of atleast 1.0 each after 24 hours exposure at room temperature. Furthermore,this invention encompasses such a vulcanized styrene butadienerubber-containing article comprising at least one silver ion controlrelease additive, such as those selected from the group consisting offillers (such as carbon black, calcium carbonate, inorganic salts,organic salts, silica, and mixtures thereof) and plasticizers (oils suchas phthalate oils and paraffinic oils). This invention also encompassesa non-silicone pre-vulcanized raw rubber formulation comprising at leastone rubber constituent, the majority of which must be a non-siliconerubber, at least one silver-based antimicrobial compound and at leastone curing compound, wherein all of said curing compound present withinsaid formulation does not include an appreciable amount of sulfur-basedcompounds, and wherein said rubber formulation optionally comprises atleast one blowing agent, at least one silver ion release controladditive, and at least one antifungal additive other than saidsilver-based antimicrobial compound. Additionally, this inventionencompasses a method of producing a vulcanized styrene butadienerubber-containing article comprising the steps of providing a rubberformulation of uncured rubber, at least one non-sulfur based curingagent, and at least one silver-based antimicrobial compound, andvulcanizing said rubber formulation at a temperature of at least 150° C.and at least at a pressure of 3 bars, wherein said rubber formulation issubstantially free from sulfur curing agent and accelerator.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The term “dimensionally stable” is intended to encompass avulcanized rubber article that is structurally able to be handledwithout disintegrating into smaller portions. Thus, the article mustexhibit some degree of structural integrity and, being a rubber, acertain degree of flexural modulus.

[0013] Such a specific antimicrobial vulcanized styrene butadienerubber-containing article has not been taught nor fairly suggestedwithin the rubber industry or prior art. As noted above, the avoidanceof sulfur-based curing agents and accelerators to any appreciable degreethus permits the retention of silver antimicrobials within the finalproduct in amounts sufficient to provide long-lasting log kill rates forStaphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa,and Escherichia coli, at the very least. Furthermore, due primarily tohigh costs, non-sulfur curing agents have not been prevalent withinvulcanized rubber formulations and articles. As such, there has been noteaching or fair suggestion of coupling non-sulfur curing agents (andmost preferably peroxide curing agents) with silver-based antimicrobialagents within pre-vulcanized rubber formulations to form effectivelyantimicrobial vulcanized rubber articles.

[0014] Additionally, certain fillers and oils (such as silica, carbonblack, stearates as fillers, and phthalate and paraffinic oils) aregenerally, although not necessarily, required to provide both flexuralmodulus and structural integrity to vulcanized rubber articles. Therubber component alone generally does not exhibit proper dimensionalstability without such additives. Surprisingly, the presence of suchadditives also provides the ability to control silver-ion release at thetarget article surface. Without intending to be bound to any specificscientific theory, it appears that such fillers as silica and such oilsas paraffinic oil (as some examples), act in such a way as to drawmoisture into the article which then transports silver ions from withinthe article to the surface. In such a situation, then, the rubberarticle may exhibit enhanced silver release resulting in higher log killrates for certain bacteria due to the presence of larger amounts ofavailable surface silver ions.

[0015] Other hydrophobic fillers, such as pigments (for example, carbonblack) and calcium carbonate appear to work in the opposite manner bykeeping water out of the target article and prevent silver-ion migrationto the article surface. Thus, the reduction of such silver-ionavailability decreases the antibacterial efficacy of the rubber article.In effect, then, the actual antibacterial efficacy of the entire rubberarticle can be controlled through the presence of certain amounts ofsuch generally required fillers and oils (some hydrophilic antistaticagents also appear to act in the same manner as silica as well).

[0016] As a result, the necessary filler and/or oil constituentsrequired to provide dimensional resiliency and/or flexural modulus (andthus actual usefulness) of the finished article serve a dual purposeheretofore unrecognized within the rubber industry. Rubber articles canbe produced with specific end-uses in mind depending upon the durationof antimicrobial activity desired through the addition of specificamounts of such additives. Again, such a targeted duration antimicrobialvulcanized article and the benefits thereof have heretofore been unknownand unrecognized within the rubber industry. These rubber components arethus hereinafter referred to as “silver ion release control additives”.

[0017] The term styrene butadiene rubber is intended to cover anystandard rubber which possesses at least a majority by weight of styrenebutadiene rubber and which must be vulcanized to provide a dimensionallystable rubber article. Styrene butadiene rubber has been utilizedpreviously within the rubber industry for a variety of applications andis generally well known and taught throughout the prior art. Suchinventive rubber formulations and cured articles should also possess achemical plasticizer which aids in the breakdown period of the elastomerduring compounding and processing (and provides flexural modulusproperties to the finished article) as well as fillers required forreinforcement (e.g. calcium carbonate, carbon black, silica, and clays).Optionally, to form a blown (foam or sponge) rubber article, a blowingagent may be added to the inventive formulation.

[0018] The rubber component or components of the inventive rubberformulation and cured article is therefore a majority of styrenebutadiene rubber. The styrene butadiene rubber may be combined withother types of rubber in order to provide different strengths,flexibilities, or other properties to the ultimate cured rubber article.Other types of rubbers include, without limitation, nitrile rubber [suchas acrylonitrile-butadiene rubber (NBR)], natural rubber, chloroprenerubber, polychloroprene rubber, ethylene propylene rubber, ethylenepropylene diene monomer (EPDM) rubber, fluoroelastomer rubber,polyurethane rubber, butyl rubber, halogenated butyl rubber [such aschlorobutyl rubber and bromobutyl rubber], isoprene rubber,epichlorohydrin rubber, polyacrylate rubber, chlorinated polyethylenerubber, hydrogenated NBR, carboxylated NBR, polybutadiene rubber, andthe like.

[0019] Although the presence of silicone rubber is discouraged withinthe inventive formulation, there remains the possibility of addingcertain low amounts of such specific unvulcanized rubber componentswithout adversely affecting the overall antimicrobial rubber formulationitself. Thus, up to 25% by total weight of the formulation may besilicone rubber; however, the vast majority of the rubber formulationmust be non-silicone rubber. Furthermore, the non-silicone rubberportion must not possess an appreciable amount of sulfur-based curingagent or residue (in the finished article) and thus must be vulcanizedthrough curing with primarily non-sulfur-based compounds (such asperoxides, resins, and/or metal oxides, for example). The rubbercomponent is present in an amount of from about 10 to about 1,000 partsof the entire composition, more preferably from about 50 to about 500parts, and most preferably from about 70 to about 200 parts of theentire composition. Thus, with a total number of parts between about 100and about 2,000 parts throughout the target vulcanized rubber article,the rubber constitutes from about 25 to about 70% of the percentage byparts of the entire article. The remainder comprises additives such asfillers, oils, curing agents, the desired antimicrobial agents, optionalblowing agents, and the like (as discussed more thoroughly below).

[0020] The antimicrobial agent of the inventive raw rubber formulationmay be of any standard silver-based compounds. Such compounds, incontrast with organic types, such as triclosan, for example, do notexhibit low thermal stability and typically remain within the targetmatrix or substrate at different temperatures. Thus, such anantimicrobial is more easily controlled, as discussed above, for surfacerelease as desired. Such agents include, without limitation, silversalts, silver oxides, elemental silver, and, most preferably ionexchange, glass, and/or zeolite compounds. Of even greater preferenceare silver-based ion exchange compounds for this purpose due to the lowlevels of discoloration and enhanced durability in the final productprovided by such compounds, the efficacy provided to the finalformulation with such a compound, and the ease of manufacture permittedwith such specific compounds. Thus, the antimicrobial agent of thisinvention may be any type which imparts the desired log kill rates aspreviously discussed to Staphylococcus aureus, Klebsiella pneumoniae,Escherichia coli, and Pseudomonas aeruginosa, as merely representativeorganisms. Furthermore, such antimicrobial compounds must be able towithstand elevated processing temperatures for successful incorporationwithin the target non-sulfur (peroxide, for example) cured styrenebutadiene rubber-containing articles. Again, such antimicrobial agentscomprise, preferably, silver-containing ion exchange, glass, and/orzeolite compounds. Most preferably, such a compound is a silver-basedion-exchange compound and particularly does not include any addedorganic bactericide compounds (thereby not permitting a release ofvolatile organic compounds into the atmosphere during processing at hightemperatures, etc.).

[0021] The preferred silver-based ion exchange material is anantimicrobial silver zirconium phosphate available from Milliken &Company, under the trade name ALPHASAN®. Such compounds are available indifferent silver ion concentrations as well as mixtures with zinc oxide.Thus, different compounds of from about 0.01 to 15% of silver ionconcentration, more preferably from about 3 to about 10%, and mostpreferably amounts of about 10% by total amount of components (e.g. ofthe total amount of silver ions and zirconium phosphate) are possible.Other potentially preferred silver-containing solid inorganicantimicrobials in this invention are silver-substituted zeoliteavailable from Sinanen under the tradename ZEOMIC®, or asilver-substituted glass available from Ishizuka Glass under thetradename IONPURE®, which may be utilized either in addition to or as asubstitute for the preferred species. Other possible compounds, againwithout limitation, are silver-based materials such as MICROFREE®,available from DuPont, as well as JMAC®, available from Johnson Mathey.

[0022] Generally, such an antimicrobial compound is added to a rubberformulation in an amount of from about 0.1 to 10% by total weight of theparticular total rubber formulation, preferably from about 0.1 to about8%, more preferably from about 0.1 to about 5%, and most preferablyabout 5%.

[0023] Furthermore, with regard to silver-based inorganic antimicrobialmaterials, these particular antimicrobial rubber articles are shown tobe particularly suitable for the desired high levels of efficacy anddurability required of such articles. It has been found that certainsilver-based ion exchange compounds, such as ALPHASAN® brandantimicrobials available from Milliken & Company, (U.S. Pat. No.5,926,238, U.S. Pat. No. 5,441,717, U.S. Pat. No. 5,698,229 to ToagoseiChemical Industry Inc.), exhibit impressive bio-efficacy. After a periodof time, alternative antimicrobial compounds (e.g. triclosan, microchek,OBPA, Zn-omadine) initially suffer from decomposition under the highprocessing temperatures, which is followed by depletion of the biocidethrough leaching into the surrounding environment and finally throughdepleted bactericidal activity. However, silver-containing ion exchange,glass, and/or zeolite compounds do not suffer from these shortcomings.Such antimicrobial agents exhibit high temperature stability (>1000°C.), do not leach into the environment and provide substantial amountsof the oligodynamic silver ion to provide for the desired extensivedurability.

[0024] The inventive antimicrobial articles should exhibit an acceptablelog kill rate after 24 hours for Staphylococcus aureus when tested inaccordance with the ATCC Test Method 6538 and for Klebsiella pneumoniaewhen tested in accordance with ATCC Test Method 4352. Such an acceptablelevel log kill rate is tested for S. aureus or K. pneumoniae of at least0.1 increase over baseline (represented by the control,antimicrobial-free vulcanized rubber article). Alternatively, anacceptable level will exist if the log kill rate is greater than the logkill rate for non-treated (i.e., no solid inorganic antimicrobial added)rubber articles (such as about 0.5 log kill rate increase over control,antimicrobial-free vulcanized rubber article). Preferably, these logkill rate baseline increases are at least 0.3 and 0.3, respectively forS. aureus and K. pneumoniae; more preferably these log kill rates are0.5 and 0.5, respectively; and most preferably these are 1.0 and 1.0,respectively. Of course, the high end of such log kill rates are muchhigher than the baseline, on the magnitude of 5.0 (99.999% kill rate).Any rate in between is thus, of course, acceptable as well.

[0025] However, log kill rates which are negative in number are alsoacceptable for this invention as long as such measurements are betterthan that recorded for correlated non-treated rubber articles. In suchan instance, the antimicrobial material present within the rubberarticle at least exhibits a hindrance to microbe growth. Furthermore,such rubber articles should exhibit log kill rates of the same degreefor other types of bacteria, such as, Pseudomonas aeruginosa andEscherichia coli.

[0026] It is also contemplated within this invention that the finishedinventive articles may provide antifungal benefits as well asantibacterial characteristics. Such versatility is rare amongantibacterial compounds; however, without intending to be limited to anyparticular scientific theory, it appears that the silver ions, andparticularly the silver ions present at the article surface in greatabundance, provide excellent antifungal properties. For example, it isbelieved that this inventive rubber formulation should provide fungalkill durability of at least 15 sequential days for such organisms asAspergillus niger and possibly for mixtures of fungi includingAspergillus niger ATCC 6275, Paecilomyces variotii ATCC 18502, andTrichoderma virens ATCC 9645, when tested in accordance with Test MethodISO 846. In order to provide a greater array of potential antifungalbenefits, other compounds may be incorporated within the targetpre-vulcanized rubber formulation (and subsequent article), such as zincoxide, as one example.

[0027] Of great importance to the effectiveness of the inventivearticles in terms of antimicrobial and antifungal activity is theomission of deleterious amounts of sulfur-based curing agents,accelerators, and additives which bind silver (such as barium sulfatefiller) from the rubber article. As noted above, it is believed, withoutintending to be bound to any specific scientific theory, that sulfurreacts with the preferred silver-based antimicrobials and irreversiblybinds the silver ions (as silver sulfides, for example) within therubber composition and/or article itself. As such, the resultant silversulfides, etc., are ineffective as antimicrobial agents and theirpresence renders the final product antimicrobially inactive.

[0028] Thus, it has been necessary to produce a vulcanized rubberarticle lacking any appreciable amount of sulfur curing agents andaccelerators therein. It should be appreciated that the term“appreciable amount” permits a small amount to be present. It has beenfound that, as a molar ratio, a 1:1 ratio (and above) between sulfurmolar presence and silver molar presence results in a clear loss ofantimicrobial activity within the desired ultimate vulcanized article.However, greater molar amounts of silver in relation to sulfur provideat least some antimicrobial properties to the desired article. A molarratio range of from about 0.25:1 to about 0.000000001:1 of sulfur tosilver ions is thus at least acceptable. The primary curing agent,however, must be of non-sulfur nature (and is preferably, though notnecessarily, a peroxide-based compound) in order to provide the desiredantimicrobial activity for the subject rubber. Furthermore, the primarycuring agent may be already incorporated into the unvulcanized rubberduring the manufacturing process.

[0029] Although peroxide curing agents have been utilized forvulcanization of rubber previously, such a different type of curingagent is not widely utilized as a suitable vulcanization catalyst forrubber for a number of reasons. Foremost, such curing agents are muchmore costly than standard sulfur-based agents and thus the utilizationof such peroxides, and the like, as a replacement for the sulfur-basedcompounds have been rather limited to mostly silicone-based rubbers or,at the very least, non-antibacterial rubber articles. However, due tothe problems associated with antimicrobial activity when such compoundsare reacted with sulfur-based curing agents, alternatives to suchsulfur-based cured articles was to permit utilization of such effectiveantimicrobial compounds within raw and vulcanized rubber for long-termhigh log kill rate effects. Thus, although non-sulfur-based compoundsare not readily utilized within the non-silicone industry asvulcanization curing agents, utilization of such curing agents wasnecessary to provide an effective, ultimate antimicrobial vulcanizedrubber article.

[0030] Surprisingly, it has now been found that the inventive rubberarticles listed above are available without such sulfur-based curingagents in any appreciable amounts; most importantly, with theintroduction of certain additives, the structural integrity and/orflexural modulus of the rubber formulation is improved to an acceptablelevel, and the efficacy of the antimicrobial components can becontrolled simultaneously.

[0031] Thus, the curing agent present within the raw rubber formulationto be vulcanized to form the inventive article must be at least amajority, and preferably at least about 75% by weight of anon-sulfur-based curing agent. As discussed above, traditional sulfurand sulfur-based catalysts will not work with the inventiveantimicrobial formulations due to chemical reactions between the sulfuratoms and the biocidal Ag+ ion. However, non-sulfur-based catalystsprovide effective curing for the inventive raw rubber formulations suchas, for example, peroxide curing systems and other oxide curing systems.Peroxide curing systems include organic peroxides such as dicumylperoxide, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane,di-(t-butyl-peroxy-isopropyl)benzene,di-(t-butyl-peroxy-trimethyl)-cyclohexane, and the like, as well asinorganic peroxides. Oxides include, for example, zinc oxide. Such acuring agent should be present in amount of from about 0.5 to about 100parts per hundred parts of rubber (pphr), more preferably from about 1to about 50 pphr, and most preferably from about 1 to about 10 pphr, alleither as one curing agent alone, or as the combination of any number ofdifferent types of curing agents.

[0032] Other additives present within the inventive vulcanized rubberarticle may include any of the aforementioned silver ion release controladditives, accelerators, accelerator activators, antidegradants,softeners, abrasives, colorants, flame retardants, homogenizing agents,internal lubricants, and deodorants. Such components should be present,if at all, in rather low amounts, of from about 0.1 to about 50 pphr.

[0033] It has further been unexpectedly determined that a substantialincrease in the antibacterial and antifungal efficacy is provided uponwashing the finished inventive article. Abrading the surface of such anarticle permits increases in such characteristics due to an increase inAg+ release; however, industrial laundering of certain rubber products(mats, and the like) provide improved antimicrobial, antifungal, etc.,efficacy through a simple washing. In fact, such an increase maysteadily improve with greater numbers of consistent washes such that arubber article, as first vulcanized, may exhibit lower overallantibacterial and antifungal activity than one that has been washed one,two, three, and up to at least 20 times (in a standard industrial rotarywashing machine). Such a surprising benefit permits utilization of suchrubber articles as floor coverings (mats, as one example, such as thosewith carpeted portions or those which are rubber alone; particularlyfoamed rubber mats for antifatigue properties and reduced specificgravity so as to reduce the chances of machinery damage during suchindustrial rotary launderings and dryings), and other articles which canbe easily washed within standard laundry machines.

[0034] Furthermore, as alluded to above, friction with the subjectrubber article surface can remove very slight layers of rubber from thearticle surface thereby permitting “fresh” silver-comprisingcrystallites at the surface to act as desired in their antibacterialand/or antifungal capacities. Basically, then, the inventive articleproduced from the inventive raw rubber formulation exhibits an evendispersion of antimicrobial particles throughout the entire rubberarticle. Such an even dispersion of the biocide throughout the rubberarticle thus provides a reservoir of fresh crystallites containing thebiocidal metallic ion. As layers of the rubber are worn and abradedaway, antimicrobial particles containing untapped silver ions becomeavailable.

[0035] The preferred peroxide cured rubber articles of this inventioncontaining the antimicrobial agent can be processed into rubber articleswhich exhibit excellent antimicrobial qualities as well as antimicrobialefficiency throughout the rubber article's lifetime. Examples of othersuch rubber articles encompassed within this invention include, but arenot limited to, hard rubber mats, static dissipative rubber mats,anti-fatigue rubber mats, rubber mats which include a face fiber, rubberlink mats, rubber seals and gaskets, rubber medical goods, rubbergloves, rubber medical devices, rubber conveyor belts, rubber belts andrubber wheels used in food processing, rubber clothing, rubber shoes,rubber boots, rubber tubing, and rubber automotive fuel hoses. Suchinventive formulations may also be incorporated into a multilayeredrubber article in which the antimicrobial agent can be incorporated intoany surface layer and still provide the desired antimicrobialefficiency.

[0036] Of particular interest is the formation of multilayered rubberarticles wherein at least one of such rubber layers exhibits the desiredantimicrobial activity and thus is made from an inventive styrenebutadiene rubber-containing article. Such layered articles may beadhered together through co-vulcanization, gluing, and the like.Furthermore, layers of other types of materials may be placed betweenrubber layers as well to provide, as one non-limiting property, betterstructural stability to the desired multilayered article.

[0037] The non-limiting, preferred embodiments of these rubberformulations and articles are discussed in greater detail below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Inventive Raw Rubber Formulations

(Inventive) Rubber Base Formulation 1

[0039] Component Amount Styrene Butadiene Rubber (SBR 1502 from 100parts Enichem) Mistron Vapour (magnesium silicate filler from 25 pphrLuzanac) FEF N550 (Cabot carbon black filler) 25 pphr Brisol 300(available from AKM) 20 pphr Zinc Oxide Active (available from Bayer) 5pphr Aflux 54 (pentaerythritol tetrastearate process aid from 2 pphrRhein-chemie) di-(tert-butyl-peroxy-isopropyl)benzene (14/40 from 2 pphrAKM) di-(tert-butyl-peroxy-trimethyl)cyclohexane (29/40 2 pphr from AKM)CPL (antioxidant available from AKM) 1 pphr Stearic Acid (available fromAKM) 0.5 pphr Antimicrobial (ALPHASAN ® RC2000 from 5% by weightMilliken & Company)

[0040] This inventive raw rubber formulation was created using ALPHASAN®RC2000 (available from Milliken & Company), a silver ion-exchangezirconium phosphate salt, exhibiting 10% Ag+ concentration and includingAg_(x)Na_(y)H_(x)Zr₂(PO₄)₃, where x+y+z=1, as other components (% byweight).

[0041] The compounding of ingredients within each formulation can becarried out in an open mill, an internal mixer, or an extruder whereintensive mixing within the polymer matrix of each component will takeplace. During the mixing operation, the control of temperature rise, dueto high shear incorporation of the ingredients, is crucial to ensurethat pre-vulcanization (scorch) does not take place during processing.Generally, a maximum temperature of 120° C. is reached on single stage(pass) mixing through an internal mixer. The compounds can be furtherprocessed after mixing into specific forms to allow adequatepresentation for manufacturing into products. This could be calendering,extrusion, granulation/pelletization, strip form, fabrication andpreforming into specific shaped blanks.

[0042] The vulcanization of the compounds can be in the form of molding(compression, transfer, injection), continuous extrusion (LCM, UHF[wherepermissible], autoclave and hot air), and coatings. The vulcanization(cure) temperatures can range from 150° C. to 250° C. In this specificsituation, the rubber articles were calendared into rough mat structuresand then subjected to vulcanization under high temperature and pressure.

[0043] Testing of Vulcanized Rubber Articles

[0044] The antimicrobial tests followed were ATCC Test Method 6538 forStaphylococcus aureus and ATCC Test Method 4352 for Klebsiellapneumoniae. The internal control was created using a polypropyleneplaque. Rubber Formulation 1 exhibited a log kill reduction of 2.98 forS. aureus and 3.52 for K. pneumoniae. The maximum log kill reductionvalues obtained were 2.98 for S. aureus and 3.52 for K. pneumoniae.

[0045] Thus, the inventive formulation provided an inventive vulcanizedrubber article that exhibited improved antimicrobial activity over thesame formulation without any antimicrobial compounds present.Specifically, the SBR article achieved maximum log kill reduction forboth S. aureus and K. pneumoniae.

[0046] The inventive article was also tested for silver ion elution atthe article surface both before and after exposure of the article torepeated standard industrial wash cycles (35 lb. loads). To measureeluted silver, the target article was immersed in an aqueous saltextraction solution (sodium phosphate) for 24 hours; the extract wasthen analyzed by inductively coupled plasma measurements for ameasurement of available silver removed from the article surface. Theresults are as follows:

Experimental Table 1 Available Surface Silver Ions of Inventive RubberArticle

[0047] ppb Sample ID Rubber Type wash cycles of bio-available Ag⁺/cm²Formulation 1 SBR 0 wash 0.356 Formulation 1 SBR 20 wash 1.571

[0048] Thus, surprisingly, the amount of available silver ions increaseddramatically not just from the article's finished state, but also up to(and beyond) twenty standard washes. Such an unexpected benefit thusprovides the user with an antimicrobial structure, such as a mat, thatactually increases its antimicrobial efficacy during use.

[0049] Having described the invention in detail, it is obvious that oneskilled in the art will be able to make variations and modificationsthereto without departing from the scope of the present invention.Accordingly, the scope of the present invention should be determinedonly by the claims appended hereto.

We claim:
 1. A dimensionally stable vulcanized rubber article comprisingat least a majority of styrene butadiene rubber and at least onesilver-based antimicrobial compound, wherein said rubber articleexhibits log kill rates in accordance with ATCC Test Method 6538 forStaphylococcus aureus and ATCC Test Method 4352 for Klebsiellapneumoniae of at least 1.0 each after 24 hours exposure at roomtemperature, and wherein said article optionally comprises at least onesilver ion release control additive, and at least one antifungaladditive other than said silver-based antimicrobial compound.
 2. Therubber article of claim 1 wherein said article exhibits log kill ratesfor Staphylococcus aureus and Klebsiella pneumoniae of at least 2.0 eachafter 24 hours exposure at room temperature.
 3. The rubber article ofclaim 1 wherein said silver-based antimicrobial compound is selectedfrom the group consisting of elemental silver, silver oxides, silversalts, silver ion exchange compounds, silver zeolites, silver glasses,and any mixtures thereof.
 4. The rubber article of claim 2 wherein saidsilver-based antimicrobial compound is selected from the groupconsisting of elemental silver, silver oxides, silver salts, silver ionexchange compounds, silver zeolites, silver glasses, and any mixturesthereof.
 5. The rubber article of claim 1 wherein said at least onesilver ion control release additive is present.
 6. The rubber article ofclaim 1 wherein said antifungal additive other than said silver-basedantimicrobial compound is present.
 7. The rubber article of claim 5wherein said at least one silver ion control release additive isselected from the group consisting of fillers, oils, pigments, salts,antistatic agents, and any mixtures thereof.
 8. The rubber article ofclaim 7 wherein said at least one silver ion control release additive isa hydrophilic filler selected from the group consisting of silica,stearates, and any mixtures thereof.
 9. The rubber article of claim 7further comprising at least one hydrophilic oil selected from the groupconsisting of paraffinic oil, phthalate oil, and any mixtures thereof.10. The rubber article of claim 1 wherein said article is a matstructure.
 11. An antimicrobial vulcanized rubber article comprising atleast a majority of styrene butadiene rubber wherein said articleexhibits an increase in silver elution when measured first for silverelution after initial article production as compared with subsequentmeasurement for silver elution after said article is exposed to twentystandard launderings within a standard industrial rotary washingmachine.
 12. A pre-vulcanized rubber formulation comprising at least onerubber constituent, the majority of which must be a non-silicone rubber,at least one silver-based antimicrobial compound and at least one curingcompound, wherein all of said curing compound present within saidformulation does not include an appreciable amount of sulfur-basedcompounds, and wherein said rubber formulation optionally comprises atleast one blowing agent, at least one silver ion release controladditive, and at least one antifungal additive other than saidsilver-based antimicrobial compound.
 13. The rubber formulation of claim12 wherein said rubber constituent is styrene butadiene rubber.
 14. Therubber formulation of claim 12 wherein said silver-based antimicrobialcompound is selected from the group consisting of elemental silver,silver oxides, silver salts, silver ion exchange compounds, silverzeolites, silver glasses, and any mixtures thereof.
 15. The rubberformulation of claim 13 wherein said silver-based antimicrobial compoundis selected from the group consisting of elemental silver, silveroxides, silver salts, silver ion exchange compounds, silver zeolites,silver glasses, and any mixtures thereof.
 16. The rubber formulation ofclaim 12, wherein said curing compound comprises a majority amount byweight of at least one peroxide.
 17. The rubber formulation of claim 16,wherein said peroxide is an organic peroxide.
 18. The rubber formulationof claim 13, wherein said curing compound comprises a majority amount byweight of at least one peroxide.
 19. The rubber formulation of claim 18,wherein said peroxide is an organic peroxide.
 20. The rubber formulationof claim 14, wherein said curing compound comprises a majority amount byweight of at least one peroxide.
 21. The rubber formulation of claim 20,wherein said peroxide is an organic peroxide.
 22. The rubber formulationof claim 15, wherein said curing compound comprises a majority amount byweight of at least one peroxide.
 23. The rubber formulation of claim 22,wherein said peroxide is an organic peroxide.
 24. The rubber formulationof claim 12 wherein said at least one blowing agent is present.
 25. Therubber formulation of claim 12 wherein said at least one silver ioncontrol release additive is present.
 26. The rubber formulation of claim12 wherein said antifungal additive other than said silver-basedantimicrobial compound is present.
 27. The rubber formulation of claim25 wherein said at least one silver ion control release additive isselected from the group consisting of fillers, oils, pigments, salts,antistatic agents, and any mixtures thereof.
 28. The rubber formulationof claim 27 wherein said at least one silver ion control releaseadditive is a hydrophilic filler selected from the group consisting ofsilica, stearates, and any mixtures thereof.
 29. The rubber formulationof claim 27 further comprising at least one oil selected from the groupconsisting of paraffinic oil, phthalate oil, and any mixtures thereof.30. A method of producing a rubber article exhibiting long-lasting,regenerable antimicrobial characteristics, comprising the steps ofcompounding together the unvulcanized rubber formulation of claim 12,molding said rubber formulation into a preselected shape, andvulcanizing said rubber formulation under high pressure and exposure tohigh temperature.
 31. A method of producing a rubber article exhibitinglong-lasting, regenerable antimicrobial characteristics, comprising thesteps of compounding together the unvulcanized rubber formulation ofclaim 13, molding said rubber formulation into a preselected shape, andvulcanizing said rubber formulation under high pressure and exposure tohigh temperature.
 32. A method of producing a rubber article exhibitinglong-lasting, regenerable antimicrobial characteristics, comprising thesteps of compounding together the unvulcanized rubber formulation ofclaim 24, molding said rubber formulation into a preselected shape, andvulcanizing said rubber formulation under high pressure and exposure tohigh temperature.
 33. A method of producing a rubber article exhibitinglong-lasting, regenerable antimicrobial characteristics, comprising thesteps of compounding together the unvulcanized rubber formulation ofclaim 25, molding said rubber formulation into a preselected shape, andvulcanizing said rubber formulation under high pressure and exposure tohigh temperature.
 34. A method of producing a rubber article exhibitinglong-lasting, regenerable antimicrobial characteristics, comprising thesteps of compounding together the unvulcanized rubber formulation ofclaim 26, molding said rubber formulation into a preselected shape, andvulcanizing said rubber formulation under high pressure and exposure tohigh temperature.
 35. A rubber composition comprising at least onerubber component, at least one peroxide curing agent, and at least onesilver-based antimicrobial agent, and optionally comprising at least oneblowing agent, at least one silver ion release control additive, and atleast one antifungal additive other than said silver-based antimicrobialcompound.